1. Field of the Invention
The present invention relates to a fixing device, an image forming apparatus, and a computer-readable recording medium storing a program.
2. Description of Related Art
As an example of conventional fixing devices used for an electrophotographic image forming apparatus such as a copier, a printer, and a FAX, a fixing device (of heat roller type) has been known which includes a heating roller having a rubber layer at a surface layer and a heat source provided inside, and a pressure roller which makes pressure contact with the heating roller. In this fixing device, in a printing operation, a toner receiving article (for example, recording paper) on which an unfixed toner image is formed is conveyed and heated by sandwiching the toner receiving article between the rollers to fix the toner image on the recording paper.
In addition, for example, a belt type fixing device is known which includes a heating roller having a heat source provided inside, an upper pressure roller, a thin endless belt wound around the rollers (hereinafter referred to as “fixing belt”), and a lower pressure roller which makes pressure contact with the upper pressure roller. In this belt type fixing device, in the printing operation, a recording sheet is conveyed by sandwiching the sheet between the upper and lower pressure rollers while heating the sheet through the fixing belt to fix the toner image on the recording sheet. In the belt type fixing device, the heat value which is consumed when heating the thin fixing belt is smaller than the heat value which is consumed when heating a rubber layer having a small thermal conductivity in heat roller type fixing devices. In view of this, the belt type fixing device is advantageous in terms of energy saving.
Incidentally, when the upper pressure roller is cold in the printing operation, the heat value is insufficient in the case where the heat value of the fixing belt heated by the heating roller is used not only for melting a toner on the recording paper at the fixing nip but also for heating the upper pressure roller and a plurality of sheets are printed. As a result, the fixing temperature required for fixing the toner image may not be obtained, and fixation failure may be caused. In view of this, in the operations other than the printing operation such as the warming-up operation after power is switched on and the standby mode before the printing, the upper pressure roller is always or intermittently rotated, and the heat value of the fixing belt heated by the heating roller is used for warming-up of the upper pressure roller.
However, since the upper pressure roller is in pressure contact with the lower pressure roller, the lower pressure roller removes the heat for warming-up from the upper pressure roller, and the lower pressure roller is heated. In this manner, the heat value is wastefully consumed. In addition, when the heat for warming-up is removed and the heat value of the upper pressure roller becomes insufficient, the fixing temperature may not be obtained, and fixation failure may be caused.
Japanese Patent Application Laid-Open No. 2005-215580 discloses a technique in which, in the standby mode, the lower pressure roller is released, and is not allowed to make pressure contact with the upper pressure roller.
At the time of warming-up of the upper pressure roller, by preventing the lower pressure roller from making pressure contact with the upper pressure roller as the technique disclosed in Japanese Patent Application Laid-Open No. 2005-215580, the heat for warming-up is not removed by the lower pressure roller, and the heat value of the upper pressure roller does not become insufficient. Consequently, stable fixation performance can be achieved without reducing the fixing temperature. However, even when the lower pressure roller is prevented from making pressure contact with the upper pressure roller, the upper pressure roller is heated through the fixing belt heated by the heating roller, and as a result the efficiency of the warming-up of the upper pressure roller is disadvantageously reduced.